Parkinson's disease (PD) is the second most common aging-related neurodegenerative disorder. It is characterized by selective loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) of the brain. Despite intense research in the past decade, mechanisms underlying the selective dopaminergic neuron death have not been well defined. Although inhibition of complex I activity is one of the leading hypotheses for dopaminergic neuron death associated with PD, this hypothesis has not been tested genetically. Our collaborator, Dr. Richard Palmiter, recently generated a transgenic mouse strain lacking functional Ndufs4, a gene encoding a subunit required for complete assembly and function of complex I. Our preliminary data demonstrate that deletion of the Ndufs4 gene abolishes complex I activity in midbrain mesencephalic neurons cultured from embryonic day (E) 14 mice. However, dopaminergic neurons in these cultures appeared normal and healthy with no decrease in survival during culture compared to neurons from wild-type mice. In this proposal, we will use conditional Ndufs4 knock out mice to determine if complex I inhibition is a main mechanism underlying dopaminergic neuron degeneration induced by the MPTP and rotenone models of PD in vivo and to elucidate complex I inhibition-independent mechanisms of cell death.